A photoconductive process of an electrophotographic light sensitive element comprises:
(1) a step of generating electric charges due to exposure to light, and, PA1 (2) a step of transporting the charges. PA1 (1) electrophotographic light sensitive elements are electrically charged to an appropriate potential in the dark; PA1 (2) discipitation of charges is less in the dark; PA1 (3) charges can be readily discharged by light exposure; etc.
A typical example of a material in which steps (1) and (2) are carried out using the same substance such as a selenium light sensitive plate. On the other hand, as an example of materials in which steps (1) and (2) are effected by different substances, the combination of amorphous selenium and poly-N-vinyl-carbazole is well known. A method wherein steps (1) and (2) are effected by different substances is advantageous in that the range for selecting raw materials used for electrophotographic light sensitive elements is broadened, electrophotographic properties such as sensitivity, acceptable electric potentials, etc. of electrophotographic light sensitive elements are improved with the broadened range, and substances convenient for preparation of coating of electrophotographic light sensitive elements can be chosen over a wide range.
Typical examples of photoconductive substances of electrophotographic light sensitive elements which have been heretofore used in electrophography include inorganic substances such as selenium, cadmium sulfide, zinc oxide, etc.
Carlson has already clarified in U.S. Pat. No. 2,297,691, that a photoconductive substance comprising a support having coated thereon a material which is insulate in the dark and changes electrical resistivity depending upon the amount of imagewise exposure can be employed in electrophotography. First, surface charges are uniformly imparted to the photoconductive substance in the dark, generally after dark adaptation during a suitable period of time. Next, the substance is imagewise exposed by an irradiated pattern which has the effect of reducing the surface charges depending upon relative energy contained in various parts of the irradiated pattern. The surface charges of electrostatic latent images thus remaining at the surface of a layer of the photoconductive substance (electrophotographic light sensitive layer) are converted into visible images by bringing the surface into contact with a suitable indicator for detecting charges, i.e., a toner.
Toners are contained in an insulating solution or a dry carrier; in any case, toners can be adhered to the surface of an electrophotographic light sensitive layer depending upon charge pattern. The thus adhered indicator can be fixed by conventional means such as heat, pressure, solvent vapor, etc. Further, electrostatic latent images can be transferred to a second support (e.g., paper, film, etc.). In a similar manner, it is possible to transfer electrostatic latent images to a second support and develop there. Electrophotography is one of the image-forming methods designed so as to form images as described above.
In such electrophotography, the following basic properties are required for electrophotographic light sensitive elements:
Inorganic substances described above that are conventionally employed have many advantages but also have various disadvantages. For example, selenium which is now widely used satisfies the requirements (1) to (3) described above but it is not desirable because it is difficult to set forth conditions for production thereof, production costs are high, it is difficult to finish up selenium to a belt form due to lack of flexibility, and care must be taken when handling selenium due to its sensitivity to heat and mechanical impact, etc. Cadmium sulfide or zinc oxide is dispersed in a resin as a binder and the dispersion is used as an electrophotographic light sensitive element; but due to mechanical drawbacks in smoothness, hardness, tensile strength, anti-abrasion, etc., they cannot be used repeatedly.
In recent years, electrophotographic light sensitive elements using various organic substances have been proposed to eliminate the drawbacks of such inorganic substances and, some are provided for actual use. For example, there are an electrophotographic light sensitive element comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237), an electrophotographic light sensitive element comprising poly-N-vinylcarbazole sensitized with a pyrilium salt dyestuff (U.S. Pat. No. 3,617,268), an electrophotographic light sensitive element comprising an organic pigment as a main ingredient (Japanese Patent Publication No. 11945/'81, Japanese Patent Application (OPI) Nos. 133445/'78 and 116039/'81, and U.S. Pat. No. 3,898,084) an electrophotographic light sensitive element containing as a main ingredient a co-crystal complex comprising a dye and a resin (U.S. Pat. Nos. 3,732,180 and 3,684,502), etc.
In these organic electrophotographic light sensitive elements, mechanical properties and flexibility of the aforesaid inorganic electrophotographic light sensitive elements are improved to some extent but light sensitivity is generally poor and they are unsuited for repeated use; these conventional electrophotographic light sensitive elements do not sufficiently meet the requirements for electrophotographic light sensitive elements.